In the past it was known to remotely control an operating table by utilizing a mobile transmitter and a receiver associated with the operating table. The transmitter uses, in accordance with the number of the functions of the operating table to be controlled, a number of feed-in keys combined to form a feed-in keyboard for feeding-in binary command signals in the 1-of-n code associated with the functions; a frequency generator having a plurality of inputs and controllable in frequency by feeding-in a code word corresponding to the relevant command signal and switchable on as a function of the appearance of a command signal; and a transmission convertor fed by the latter command signal and transmitting frequency signals corresponding to the command signals. The receiver has a reception convertor and means for selective amplification of the frequency signals received and for their reconversion into the command signals.
In this case the operation of a feed-in key causes the frequency generator, which is constructed as a free running oscillatory, to be connected to a capacitor, while the capacitors which can be connected by means of different keys exhibit different values, so that a different frequency is associated with each feed-in key. In the case of this arrangement accurate evaluation of the frequency impulses transmitted as to their duration is impossible due to the necessary building-up processes and to the echo effects which occur.
Furthermore, the arrangement is extremely susceptible to interference. But many such interference sources are to be found particularly in hospitals where remote control arrangements for medical appliances are principally used. For example, if the ultrasonic range is chosen for the transmission of the frequency signals, then interference signals may originate from ultrasonic cleaning machines for instruments, ultrasonically operated hand cleaning installations, high-frequency surgical applicances, ultrasonic diagnostic machines or ultrasonic bone welding appliances. Experience also shows that ultrasonic components occur in many resonance phenomena, e.g. in wind noises, in exhaust ducts or in telephone installations. The susceptibility to interference is particularly serious when, e.g. in a hospital with a variety of operating theatres, the respective operating tables are required to be served by means of similar remote control arrangements, because then each of these arrangements acts as an interference transmitter for at least those remote control arrangements in use in the adjacent rooms.
A remote control arrangement similar to the above type for television receiving sets is also known, wherein the frequency generator is constructed to generate a group frequency signal in addition to the frequency signals corresponding to the command signals as a function of the input group code word. The transmitter has an impulse generator which can be set in operation as a function of the appearance of a command signal. The group code word can be fed into the frequency generator instead of the code word corresponding to the relevant command signal as a function of the output impulses generated by the impulse generator. The receiver has a circuit which controls the emission of the command signals as a function of the alternating reception of a frequency signal corresponding to the command signal and of the group frequency signal. In this case therefore, during the duration of the imputing of a command signal into the transmitter by depressing an input key, the frequency signal is transmitted as a sequence of frequency signal impulses which alternate with group frequency signals of a group frequency different from all the frequency signals corresponding to command signals which can be generated, and both the frequency signal impulses and the group frequency signal impulses are amplified selectively in the receiver. The receiver also has a decoding circuit which when fed by frequency signal impulses at one time from a number of outputs corresponding to the number of the frequency signals which can be generated, emits command signal impulses, which when supplied with the group frequency signals to a further output emits indicator impulses indicating the reception of said group frequency signal impulses, and which when fed simultaneously by two signals of different frequency and approximately equal amplitude generates no output signals. The receiver further has means to emit the command signal corresponding to the respective command signal impulses as a function of the appearance of the indicator impulses, so that the emission of the command signal to the associated appliance occurs only when indicator impulses and command signal impulses are emitted alternately by the decoding circuit. The freedom from interference is already quite substantially improved in this case compared to the aforementioned known method. Nevertheless, in many cases this freedom from interference still does not fulfil the desiderata which must be imposed as to safety in the control of medical appliances. This is due to the fact that the group frequency different from the frequencies corresponding to the command signals encounters different transmission characteristics along the transmission path, so that the amplitudes of an interference radiation at the receiver may be appreciably smaller than the amplitudes of the group frequency signal impulses received and appreciably greater than the amplitudes of the frequency signal impulses received corresponding to command signals. In this case the decoding circuit of the receiver alternately generates indicator impulses and command signal impulses corresponding to the interference radiation, whereby a false control of the appliance occurs.